Step 1In a large bowl, whisk together the tahini and honey.
Step 2In a medium bowl, add the vanilla seeds to the cream and use a whisk or hand mixer to whip to medium-soft peaks. Vigorously whisk one-third of the whipped cream into the tahini-honey mixture to lighten it, then add the rest in two batches, gently folding each with a rubber spatula until just a few white traces of cream remain. Cover the bowl with plastic wrap and refrigerate for at least 1 hour or up to 3 days.
Step 1Combine the cacao nibs and salt in a spice grinder, small food processor or blender (or use a mortar and pestle) and process until finely ground (a few small pieces here and there are fine). Transfer to an airtight container and store for up to 2 weeks.
Step 1Heat the oven to 350 degrees. Grease the bottom and sides of an 8-inch square baking pan with butter, line the bottom and two sides with a sheet of parchment paper or foil, then grease the paper too.
Step 2Melt the butter in a small saucepan over low heat, then add the chocolate and cacao nibs and continue cooking, stirring occasionally, until the chocolate just melts. Remove the saucepan from the heat and whisk in the sugar and vanilla seeds, then whisk in the eggs until smooth. Add the flour and salt, stir until just combined, then scrape into the prepared pan.
Step 3Drizzle the tahini evenly over the top of the batter, then use a knife to lightly marble it throughout the batter. Bake for 10 minutes, then rotate the pan, and continue baking until the brownies are just set but still soft in the middle, exactly 8 minutes more. These brownies can go from just right to dry fairly quickly, so when in doubt, pull them out. You can't go wrong with a soft middle.
Step 4Transfer the pan to a wire rack and let the brownies cool completely in the pan. Using the overhang from the parchment paper or foil, carefully lift the brownies out of the pan and onto a cutting board. Cut into squares and store the brownies in an airtight container at room temperature for up to 3 days.
Step 5If serving the brownies restaurant-style, place 1 brownie on each serving plate, leaving it whole or breaking it into 4 pieces with your fingers, then top with a large dollop of the tahini whipped cream. Sprinkle with some of the cacao nib crumble before serving.